1. Field of the Invention
This invention relates to a device for conveying a mixture of air and fibers or other particles and more particularly to an improved centrifugal blower provided with a multi-dimensionally curved impeller and multi-dimensionally curved blades in an open construction.
2. Description of the Prior Art
Centrifugal blowers for conveying fiber and air mixtures have a housing with an intake disposed along the longitudinal axis of the impeller. The fiber and air mixture is drawn into the blower along the impeller's longitudinal axis and is then moved radially outward past the periphery of the impeller. The fiber-air mixture as it is moved radially outward is accelerated in a circumferential or aximuthal direction. The outlet for the blower is normally located on the housing outward from the periphery of the impeller.
Various designs of centrifugal blowers for the purpose of conveying fibrous materials and air mixtures are known in the prior art. Impeller constructions with straight blades or curved blades installed on flat impeller plates have been proposed. For example, in "Ventilatoranlagen" by Mode, 4th edition, page 82, an open impeller with a flat disk wheel has been proposed for pneumatic conveying of solids, and in "Ventilatoren" by Eck, 8th edition, page 479, an impeller with a flat disk wheel is also proposed. Blowers equipped with impellers of this construction exhibit considerable drawbacks in fiber conveying due to the large amount of noise generated as the result of the turbulence. The service life of the blades in these type of constructions is short due to the high impact load exerted on the blades by the fibers or particles. Blowers utilizing the above construction provide for a low draw-in or suction on the fiber intake. They exhibit a high consumption of energy as a result of their reduced efficiency and the unfavorable conveying of the fibers.
The inventors of the instant application recognized that these disadvantages with the prior art blowers are due to the unfavorable guidance of the flow of the fiber and air mixture. The problem is particularly acute when greater bulk weights or denser particles are used. The fibers or particles posses a low relative velocity with respect to the intake air and consequently do not follow the flow path of the air directly. The particles do not accelerate to the speed of the flowing air and more particularly to its peripheral speed component until they strike the impeller blades. Consequently, straight blades receive high impact stress from the fibers or particles and this frequently causes the blades to bend over. When the blades are curved and the plate is straight, the deflection of the fibers from the axial blower intake to the radial periphery of the impeller does not occur without a relatively large motion with respect to the intake air. The air material flow is unfavorably guided by the impeller and there is a high material load at the point of impact of the fiber on the impeller plate. This can damage both the fiber, by causing it to splinter or shorten, and the impeller plate. Moreover, when a dirty wool fiber is used, a high degree of erosion is caused at these points by the entrapped solids or sand particles in the fibers.
In order to relieve these drawbacks, it has been proposed in the past to design the blades with a low height. However, this has the disadvantage of limiting the output of the blower with a given impeller diameter and therefore opposes the construction desired in a pneumatic heavy duty installation for conveying large amounts of fibers.